US2762722A

US2762722A - Method and apparatus for coating by thermal evaporation

Info

Publication number: US2762722A
Application number: US355557A
Authority: US
Inventors: Frank K Truby
Original assignee: Bausch and Lomb Inc
Current assignee: Bausch and Lomb Inc
Priority date: 1953-05-18
Filing date: 1953-05-18
Publication date: 1956-09-11
Anticipated expiration: 1973-09-11

Description

Sept. 11, 1956 F. K. TRUBY 2,762,722

METHOD AND APPARATUS FOR COATING BY THERMAL EVAPORATION Filed May 18, 1953 .INVENTOR.

FRANK K. TRU BY ATTORNEY United States Patent Ofi 2,762,722 Patented Sept. 11, 1956 ice METHOD AND APPARATUS FOR COATING BY THERMAL EVAPORATION Frank K. Truby, Socorro, N. Mex., assignor to Bausch &

Lomb Optical Company, Rochester, N. Y., a corporation of New York Application May 18, 1953, Serial No. 355,557 14 'Claims. (Cl. 117106) This invention relates generally to evaporation coating in a vacuum and more particularly to an improved apparatus and method for depositing coatings onto a base.

The subject matter of this patent may be used by or for the United States Government for governmental purposes Without the payment of any royalties to me.

As the art of high vacuum evaporation of materials continues to develop, new requirements demand improvements over the existing devices. One big demand has been for some practical way to support within a vacuum chamber a relatively large supply of material so that practically all of it can be successfully evaporated onto a base. This problem arose because some processes require coatings of material which are several hundred quarter wave lengths thick while other processes require a thin coating of material to be applied to a relatively long, continuous strip or the like. Existing prior devices cannot meet this demand. For example, some prior devices have heating filaments disposed over the material being evaporated so that only a relatively small amount of material may be evaporated before the surface of the material becomes too remote for evaporation to continue.

Another demand has been to devise some new way to evaporate material without the evaporation heating filament excessively raising the temperature of the base. For instance, some materials deposit better on cool bases, but

due to the large amount of heat radiated from the large exposed evaporation filaments, the bases are soon heated to quite a high temperature with resulting complications.

Other materials deposit better on bases that are maintained at some constant temperature. However, the prior art evaporation filaments continually radiate heat onto the bases to destroy the constant temperature level required.

Therefore, it is an object of my invention to provide means in an evaporation apparatus for holding a relatively large supply of coating material so that substantially all of the material can be evaporated without difliculty.

Another object of my invention is to provide a new and improved apparatus for vaporizing a coating material without heating the substrate or base.

A further object of this invention .is to provide a new and improved apparatus for uniformly depositing relatively thick layers of coating material on a base.

A still further object of my invention is to provide an improved apparatus for easier and more accurate control of the deposition of a coating on a base.

Still another object of my invention is to provide an apparatus for depositing a coating that is satisfactorily durable and wear resistant for many purposes.

Another object of my invention is to provide a new method for preparing and evaporating a coating material onto a base.

These and other objects and advantages reside in certain novel features of construction, arrangement and combination of parts and in the method steps and method as will hereinafter be more fully explained and pointed out in the appended claims.

Referring to the drawings:

Fig. 1 is a sectional view of an evaporating unit embodying one form of my invention.

Fig. 2 is a top view of the new material holding device of Fig. 1

Fig. 3 is an enlarged vertical sectional view through the material holding device after some of the material has been evaporated.

A preferred form of the present invention is shown in Fig. 1 wherein similar reference characters designate corresponding parts throughout the several views. The apparatus for coating a base according to this invention comprises a stand 10 on which is positioned a bell jar or the like 11 to form a vacuum chamber 12 having a work support 13 comprising legs 14 supporting a metal plate 15 which has seats for holding bases or workpieces such as lenses 16. The vacuum chamber 12 is evacuated by means of a vacuum pump 17 connected to the chamber by means of a conduit 18.

Upstanding from the stand 10 is a pair of

insulated supports

19 and 20 holding, respectively, the ends of a continuous tungsten wire 23 having an upstanding filament or heating element 24 comprising

sides

25 and 26 terminating in a common end 27. It is to be understood that the hairpin shape of the heating element 24 is for illustration only, and other shapes that accomplish the objects of the invention are to be included within .the scope of the invention. A source of electric power is connected to the wire 23 through a pair of lead-in wires 21 entering the vacuum chamber through the sealing plugs 22 in the stand 10. A sheet of mica 28 or other insulating material is disposed on the

supports

19 and 20 and has an aperture 29 through which the heating element 24 projects. The sheet of mica 28 is disposed between the wire 23 and the bases 16 to shield the bases from the heat of the wire and to act as a support for a crucible 30;

form of my invention has compacted into the crucible 30 a suitable coating material forminga block or mass 36 having a central, vertical cavity or opening 37 extending from an orifice 38 in the top surface of the mass at the open end of the cup-shaped crucible through the block or mass 36 to an orifice 39 aligned with the aperture 34 in the bottom 33 of the crucible 30. The material used must be of a type that sublimes and is generally available in powder, granular or chunk crystal state. The powder or granular state of the material can be packed together to form a block or mass 36 which is self-rigid or it can be held in the crucible 30, as shown. The opening 37 .is formed in the mass or block 36 when the massis being compacted or the opening 37 can be drilled or forced in the block or mass after compacting. The chunk crystal form of the material is in block or mass state and must have an opening or cavity 37 drilled or formed into the body of the mass. When the block or mass 36 is made of chunk crystal, .it is self-rigid and need not be placed in the crucible 30, but it may be so disposed if desired.

The crucible 30 and its block or mass of coating material 36 is adapted to fit around the heating element 24 and to rest on the crucible bottom 33 upon the sheet of mica 28. For most materials the heating element or filament 24 extends up through the aperture 34 in the crucible and into the opening 37 in the compacted mass toa point just below the orifice 38 in the top surface thereof. However, some materials, i. e., cryolite, when heated will slowly grow over the orifice 38 of the open ing 37 of the material if the end 27 of the filament 24- is below the orifice. With this type material, the heat ing element 24 is extended slightly beyond the top surface of the block or mass of material to keep the orifice open. When the block or mass 36 is formed either of the selfrigid crystal type or of the self-rigid compacted type of material, it can be placed directly on the sheet of mica 28 with the filament 24 extending into the opening 37. The opening 37 in the block or mass is of such a size and shape that the heating element 24 is substantially coextensive with and spaced from the wall portions of said opening. In this way, the coating material will not be contaminated by contact with the filament or heating element 24.

The coating material ,36 must be of the type that sublimes, going directly from the solid to the vapor state when heated. Materials such as zinc sulfide, cryolite, chiolite and the like fall generally in the group that sublime. Since the materials sublimate, when the filament or heating element stays below the orifice of the open ing in the material, the mass 36 will be disintegrated by radiant heat in the region around the heating element 24, leaving intact the material around the orifice 38 in the end of the opening 37 until the shape of the block or mass assumes the appearance of Fig. 3 in vertical section. The orifice 38 remains substantially fixed in shape and diameter due to the end 27 of heating element 24 being disposed below the exposed surface of the block or mass so that not enough heat is available to sublimate the material around the orifice 38. This orifice 38 will act as an apertured mask or shield which always permits the same crosssectional cone or pattern of evaporated material to be projected from the block or mass along the path toward the workpieces .or bases 16. After a certain amount of use, the original orifice 38 will disintegrate somewhat whereby the orifice will be enlarged. If desired, an apertured mask may be placed on top of the crucible so that the aperture in the mask will permit the material to escape in a preselected cone or pattern shape.

When the filament or heating element 24 extends beyond the orifice 38 of the opening 37 in the material, the orifice may not develop as shown in Fig. 3, .but the evaporation will take place coextensive with the filament 24. While under this last-mentioned condition, a certain amount of control over the size of the cone of emitted material will be lacking, it will still be effective for evaporating large quantities of material.

In many prior devices, the relatively large evaporation filaments or heating elements are exposed to the base or workpiece 16 whereby the base or workpiece is continually heated'without any practical means of control. [he present invention is designed to overcome this heating by having only the small end area 27 of the filament or heating element 24 exposed to the base or workpiece 16 so that even after long periods of depositing material many wave lengths thick, the base or workpiece 16 'will not beexcessively heated by the filament or heating element 24. This removes a variable that has always been very difficult to control, especially when continuously depositing many layers of films on a base over a period of many hours, during which time the evaporation filament or heating element would continue to heat the base to a higher and higher temperature. With the large heating element no longer exposed to the bases, it is now possible with my invention to substantially maintain the bases at any desired predetermined temperature. For instance, if a base is inserted into the vacuum chamber at room temperature it will, by the use of my invention, remain substantially at room temperature and if a base is heated to a preselected temperature by heaters within the vacuum system, the temperature of the base will not be materially altered by :the evaporation filament or heating element.

When carrying out the method of my invention, the bases or lenses 16 to be coated are placed in the evacuating chamber on the support 13 within the range of the emitted coating material. In one form of my invention,

a block or mass of coating material which will sublime is packed into a crucible 30 so that it has the opening 37 running therethrough from an aperture 34 in the bottom of the crucible to an orifice 38 in the top surface of the mass. The crucible is placed in the evacuating chamber with the orifice 38 directed toward the bases 16 and with the heating element 24 within the opening 37 in the block or mass so that with certain materials the end 27 of the element is located just below the orifice 38 of the opening. After the chamber is evacuated, the radiant heat from the heating element sublimates the block or mass of material around the wall portions of the opening whereby the vapor of the sublimed material will be emitted from the orifice of the opening in the block or mass along a cone path to the base to be coated.

The crucible 30 of the present invention can be made large enough to contain a block or mass of material for depositing a coating several hundred quarter wave lengths thick. However, even if the mass of material is of the same volume as the supply of material used in prior art devices, many times more material can be successfully evaporated from the block or mass using the present invention. This is primarily because such prior art devices have the heating coil located so that the material is evaporated only from the exposed side of the material which is adjacent to the heating coil. After a time, the material close to the coil is evaporated away leaving the rest of the material too far from the heating coil to be evaporated. With the present invention, the heating element is disposed within an opening in the mass of material so that material is evaporated from substantially the entire wall portion of the opening which is adjacent to and substantially coextensive with the heating element. This means that a larger surface area of material is exposed to the heating element in the present device so that much more material is evaporated. Since the heating coil is almost completely surrounded by material, it has been found that the heat within the opening is not dissipated so rapidly. Therefore, the heat remains longer within the opening to evaporate more material from around the walls thereof. In this way, the block will be evaporated away to a greater distance from the heating element than would normally be possible.

The coatings that result from using my invention have been found to be more uniform in quality and thickness. This is believed to be due to the fact that the cone of evaporated material emitted from the orifice of the opening is homogeneously filled with particles of the coating material. Also, the spread of the cone is somewhat controlled so that only that material directed toward the lenses or bases is emitted from the block whereby such emitted material is practically all deposited with very little waste.

While the block or mass of coating material 36 has beendisclosed as having an opening 37 extending therethrough, it is to be understood that the opening may not necessarily extend through the mass but may be in the form of a pocket or cavity having a bottom. Although the heat from element 24 is electrically produced, it is to be understood that other forms of heating elements may be used within the opening in the mass of material.

From the foregoing, it will be apparent that I am able to attain the objects of my invention and provide an improved apparatus and method for depositing coatings of higher quality onto a base. The small exposed end of the heating element will not excessively heat the bases or workpieces as in prior devices. The coatings will be deposited more evenly since the cone of material emitted from the orifice of the block or mass is homogeneous. Further, the present apparatus holds material sufficient to deposit coatings many times thicker than in prior devices. And, finally, the present crucible and block or mass of material has the advantage of never warping or tilting from its preset position which is a distinct advantage over former devices, in that the material will always be directed toward the bases.

Various modifications may obviously be made without departing from the spirit of my invention as pointed out in the appended claims.

I claim:

1. A method of evaporating a material which will sublime which comprises forming in a mass of the material an opening having an internal wall and an orifice and vaporizing some of the material of the wall by heatlng primarily the internal wall of said opening whereby the vapor of the material will be emitted from the orifice of the opening.

2. A method of evaporating a material which will sublime which comprises forming in a mass of the material an opening having an internal wall and an orifice, inserting heating means within said opening in the mass to substantially surround the means with the material, and activating said heating means to sublimate only the material forming the internal wall of the opening in the mass whereby the vapor of the sublimed material will be emitted from the orifice of the opening in the mass.

3. A method of evaporating a material which will sublime comprising compacting the material in a crucible, forming in the material an opening having an internal wall and an orifice, inserting a heating element into the opening to substantially surround the element with the material and heating the element to vaporize only the material from the internal wall of the opening whereby the vapor will be emitted from the orifice of the opening.

4. In a method of depositing a coating material onto a base, the steps of compacting material which will sublime into a crucible, forming an opening through the material in alignment with an aperture in the bottom portion of the crucible, inserting a heating element through the aperture and into the opening to a point below the top surface of the material and heating the element to vaporize the material from the wall portions of the opening.

5. An apparatus for evaporation of coating materials which will sublime comprising a crucible having an aperture therein and being adapted to hold a mass of coating material, said mass having an opening therethrough aligned at one end with said aperture in the crucible, and heating means extending through said aperture and into the opening and being substantially coextensive with said opening and spaced from the wall thereof for subliming the coating material forming the wall portions of the opening.

6. Apparatus for evaporation coating of bases in a vacuum comprising a chamber adapted to be evacuated, means for evaporating coating material which will sublime in the chamber, said means comprising a cupshaped crucible having an aperture in the bottom thereof said crucible being adapted to hold a mass of material having a vertical opening extending therethrough and in alignment with said aperture, and an electrical heating element for subliming said material extending through the aperture and into the opening to a point below the top surface of the material whereby coating material from the wall of the opening is sublimed and is directed through the orifice in the top surface of the material onto the bases to be coated.

7. Apparatus for evaporation coating of a base in a vacuum comprising a chamber adapted to be evacuated, means within said chamber for evaporating a material which will sublime comprising a crucible adapted to hold a mass of material formed with an opening having an internal wall and an orifice in one side thereof and heating means located within said opening and spaced from the internal wall thereof for heating the material to produce sublimation, said heating means being substantially surrounded by the material whereby the vapor of the material will be emitted from the orifice of the opening.

8. In an evaporation apparatus having a chamber adapted to be evacuated, a support in said chamber for positioning a base within the range of an evaporated coatsurrounding a heating element with said ing material the combination with means for evaporating the coating material comprising a crucible adapted to hold a mass of coating material which will sublime, the bottom of said crucible having an aperture therein, said mass having an opening therethrough aligned at one end with said aperture, and heating means projecting through the aperture and into the opening in the mass to a point below a top surface thereof and spaced from the wall portions thereof to sublime material from said wall portions around said heating means whereby said sublimed material will be projected through an orifice of the opening in the top surface of the mass into depositing contact with the base.

9. An apparatus for the evaporation of materials which will sublime comprising means for holding a mass of coating material which is provided with an elongated opening having an internal wall and an orifice, means for holding a base to be coated in alignment with the orifice, and elongated heating means positioned within the opening and substantially coextensive therewith, said heating means being spaced from the wall of the opening and having a relatively small end portion thereof located substantially at the orifice whereby the material forming the wall of the opening will be sublimed and directed through the orifice and onto the base which receives direct heat radiation only from the exposed end portion of the heating means.

10. A method of depositing a material on a base by evaporating a material which will sublime which comprises forming in a mass of the material an elongated wall and an orifice, heating only the material formmg the wall of the opening sufiiciently to produce sublimation thereof while substantially preventing sublimation of the material at the orifice and thereby retaining the original size of the orifice through which the vapor of the sublimated material passes to the base.

11. A method of depositing a sublimable material on a base located above the material by subjecting the latter to heat from an element while substantially preventing the passage of heat from the element to the base comprising the steps of substantially surrounding the element with a mass of the material by placing the element within an opening formed in the material and having an orifice and energizing the element to sublimate the material forming the wall of the opening so that the vapor formed will pass out through the orifice of the opening and be deposited on the base.

12. A method of depositing on a base a material which will sublime comprising the steps of forming an opening having an orifice in a mass of the material, substantially material by placing the element within the opening, and energizing the heating element to sublimate the material forming the wall of the opening whereby the vapor formed will pass out of the orifice of the opening and onto a base positioned above the orifice and the amount of heat passing from the element to the base will be greatly re duced.

13. A method of depositing a material on a base by evaporating a material which will sublime which comprises forming in a mass of the material an opening having an internal wall and an orifice, heating the material forming the internal wall of the opening to produce sublimation thereof Without heating the material at the orifice enough to cause sublimation thereof whereby the vapor of the sublimated material will pass to the base through the orifice which substantially retains its original size.

14. In an apparatus of the type described having a chamber adapted to be evacuated and a support in the chamber for holding a base to be coated, the combination of means in the chamber for evaporating a coating material comprising a mass of said material which will sublime, said mass of material having an opening extending therethrough and terminating at its upper end in an orifice which is located below the base to be coated, a heating element extending into the lower end of the opening and terminating substantially at the orifice whereby primarily the wall of the opening adjacent to the element may be heated to sublimation so that the vapor will pass out through the orifice and be deposited on the base.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A METHOD OF EVAPORATING A MATERIAL WHICH WILL SUBLIME WHICH COMPRISES FORMING IN A MASS OF THE MATERIAL AN OPENING HAVING AN INTERNAL WALL AND AN ORIFICE AND VAPORIZING SOME OF THE MATERIAL OF THE WALL BY HEATING PRIMARILY THE INTERNAL WALL OF SAID OPENING WHEREBY THE VAPOR OF THE MATERIAL WILL BE EMITTED FROM THE ORIFICE OF THE OPENING.